Osteopetrosis is a heterogeneous group of disorders caused by defective bone resorption of osteoclasts. The resulting accumulation of excess bone leads to a narrowing of cranial nerve foramina and a reduction in the marrow cavity. In humans, osteopetrosis can be inherited as an autosomal recessive or autosomal dominant disease. In patients with severe congenital osteopetrosis, longevity beyond the first decade of life is rare. The genetic defect causing osteopetrosis in humans is currently unknown. It has been reported that microphthalmic mice (mi/mi) develop the symptoms of osteopetrosis as a result of a mutation in microphthalmia gene, which codes for a putative transcription factor containing a basic region-helix-loop-helix domain and a leucine zipper domain. The human homolog of this mouse microphthalmia gene, MITF (microphthalmia- associated transcription factor) gene, has been cloned and located to human chromosome 3. Our long term goal aims to understand the mechanism of human osteopetrosis and to explore a potential cure for this disease. Characterization of the MITF gene in osteopetrotic patients with respect to the potential alterations in the MITF gene structure and/or expression levels will be the first step toward our destination. Therefore, the specific aims of this proposal are, firstly, to determine the tissue- specific expression pattern of the MITF gene using RT-PCR and Southern blot analysis; secondly, to compare the expression levels of the MITF gene in osteopetrotic patients with those in normal subjects by competitive RT-PCR; thirdly, to detect potential mutations in the MITF gene in osteopetrotic patients using single strand conformation polymorphism (SSCP) analysis.